SUMMARYThe balanoid (acorn-shaped) aecium of the trellis rust,Gymnosporangium fuscum (=G. sabinae)is morphologically distinctive and ecologically unique among cedar rusts for its shape and effective mechanism of spore discharge. The hygroscopic peridia remain closed in damp air during the night, but repeatedly split open during dry and windy weather in daytime. The delicate mechanism enables trellis rust the maximum production and distribution of spores. No other plant rust is known to possess such an advanced mechanism for the discharge of aeciospores.